Presently, fabric filters are used widely by industry to remove particles from a gas. Large volumes of particle laden gas can be treated. For example, bag houses are designed to handle from about 10 to over a million cubic feet per minute of gas.
Bag houses employ a surface filter as opposed to a depth filter. A surface filter collects particles on its surface to form a cake, and this cake is periodically removed by flexing, e.g. shaking the filter. Depth filters comprise randomly entwined fine fibers. When particles contact these fibers they adhere to the fiber. Some particles collect deep within the depth filter, and consequently, it is much more difficult to clean the depth filter than the surface filter. Many depth filters are simply disposed of because they cannot be cleaned conveniently.
Because a depth filter cannot be readily cleaned it is not currently used in bag houses. However, it would be desirable to use a depth filter, because depth filters would permit the size of the bag houses to be reduced substantially. This is due to the greater dirt holding capacity of the depth filter, permitting a larger volume of gas to be processed before the pressure drop across the filter exceeds the design level for the system. Surface filters, in general, cannot withstand as large a pressure drop as depth filters.
One additional problem normally associated with bag houses is that the surface filters used are made of organic materials which cannot resist high temperatures and corrosive environments. It would be highly desirable to employ in bag houses filter media which could resist hostile environments. Presently, because the fabrics cannot resist high temperatures, hot gas must be cooled.